Lasers (light amplification by stimulated emission of radiation) require incident photons for stimulated emission and amplification. In a Fabry-Perot (FP) laser partial front and back facet mirrors result in multiple passes through the laser cavity of photons and thus amplification of these photons. However, the gain profile of an FP laser is rather broad (15-20 nm) and is highly sensitive to temperature due to energy band gap and refractive index changes. The magnitude of temperature sensitivity on gain profile center wavelength ranges from 0.5 to 0.6 nm/° C. depending on laser design.
The spectral-width and wavelength and output power of laser emission can be controlled and stabilized by providing feedback to the laser cavity. The feedback photons excite a feedback mode with a lower threshold gain than the FP mode if the feedback mode is still within the laser gain profile. By lasing prior to the FP modes, only the modes which correspond to the feedback wavelength and bandwidth are populated. This provides spectral width selection and control. If the wavelength and number of feedback photons are stable against environmental (e.g., temperature) changes, the laser center wavelength temperature, spectral width, and output power sensitivity is reduced.
State of the art optical telecommunication systems require the use of multiple stable lasers for both signal transmitters and optical amplifiers. In the case of signal transmitters, a system of lasers of differing wavelengths and/or polarizations are typically multiplexed together to increase the information carrying capacity of an optical transmission fiber. In the case of optical amplifiers, a system of lasers of differing wavelengths and/or polarizations are typically multiplexed together to provide a high-power depolarized pump source which is then used to pump an optical fiber and induce gain in the optical signals being transmitted through the fiber. As is well known in the art, reflective fiber Bragg gratings are commonly used to stabilize lasers.
Referring now to FIG. 1, there is shown a block diagram of an exemplary prior art multi-laser system 10 which provides feedback to each of a plurality of n lasers (LASER 1, 2, . . . N) 12a-12n using a Distributed Bragg Grating outside the lasers 12a-12n. The system 10 comprises the lasers 12a-12n, a plurality of n optical delays 14a-14n, a plurality of n Fiber Bragg Gratings (FBG 1, 2, . . . N)) 16a-16n, a wavelength division multiplexer (WDM) 18, an optical tap 20, and an optical power detector 22. The n lasers 12a-12n each have an output coupled to a separate input of each of the optical delays 14a-14n, respectively. An output of each of the optical delays 14a-14n is coupled to a separate input of each of the Fiber Bragg Gratings 16a-16n, respectively, and an output from each of the Fiber Bragg Gratings 16a-16n is coupled to a separate input of the WDM 18. The output from the WDM 18 is coupled to an input of the optical tap 20. The optical tap 20 has a first output coupled to the optical power detector 22 and has a second output which serves as an output of the system 10.
In operation, the laser 12a transmits an optical signal at a predetermined wavelength to the optical delay 14a which delays the output signal from the laser 12a by a predetermined amount. The output signal from optical delay 14a is received by the FBG 16a which filters the delayed input signal from the laser 12a and provides a predetermined bandwidth signal to a separate input of the WDM 18. The FBG 16a also reflects a portion of the delayed input signal from the laser 12a back towards laser 12a via the optical delay 14a such that a predetermined signal with a predetermined bandwidth is fed back to stabilize laser 12a. Each of the other lasers 12b-12n and their associated optical delays 14b-14n and FBGs 16a-16n operate in the same manner as described above for laser 12a. The WDM 18 multiplexes the n signals from the FBGs 16a-16n and generates a multiplexed output signal to the optical tap 20. The portion of the multiplexed signal directed to the optical power detector 22 is optionally available for use to control the lasers 12a-12n. 
Currently, laser feedback for power and wavelength stabilization is achieved through reflective feedback gratings (a) as an integral part of the laser in Distributed Feedback Lasers (DFB), or Distributed Bragg Reflector Lasers (DBR), or (b) as a discrete component (generally a Fiber Bragg Grating) generally placed outside the laser's coherence length. The problem with typical prior art systems is the use of Fiber Bragg Gratings (FBGs) external to a laser cavity for both the filtering and reflecting feedback mechanism. Such a configuration use limits the methods that can be used for stabilizing lasers. The use of FBGs for filtering and feedback produces overall losses related to the feedback and can cause system instability related to differential drifts in the multiplexing function compared to the filtering or reflection function.
It is desirable to provide feedback mechanisms for stabilizing the lasing wavelength and controlling the spectral width of lasers, such as semiconductor lasers, and optical radiation sources in a laser system which provides for more flexibility in usable components and optical radiation sources, optical feedback with reduced overall loss, and improved production yield potential.